1. Field of the Invention
The present invention relates to monitoring devices. The invention more specifically relates to monitoring devices comprising a reader and a data carrier, adapted for wireless data and power transmission. The invention also relates to a method for controlling wireless power transmission in such a device.
2. The Prior Art
In the context of the present disclosure a monitoring device should be understood as a small device designed to be worn by a human user for continuous surveillance of a specific medical condition in the user. The monitoring device consists of a reader and a data carrier.
The reader is powered by an internal primary energy source, such as a battery. The data carrier does not have its own primary source of energy. Therefore the data carrier relies on a wireless inductive power transmission from the reader. A transmitter coil in the reader and a receiver coil in the data carrier together form a system of magnetically coupled inductors. The basic principle relies on generating an alternating current in the transmitter coil. The current in the transmitter coil generates a magnetic field which induces a current in the receiver coil. The current in the receiver coil is used to power the data carrier.
The data carrier has monitoring means, such as electrodes, for measuring EEG signals in the human user of the monitoring device. The data collected by the monitoring means are preprocessed by data processing means and wirelessly transmitted to the reader for further processing. The reader receives data from the data carrier through load-modulation of the data carrier. In a typical application the further processing in the reader includes determining if a specific medical condition has occurred in the user and alerting the user of this condition. Hereby the size and power consumption of the data carrier can be kept small because the battery and the main part of the signal processing is placed in the reader. This is advantageous in that it makes the data carrier feasible for implantation in the human user. It is especially advantageous to have the data carrier subcutaneously implanted with respect to measurement of EEG signals.
Efficient operation of the monitoring device requires that the inductive power transmission from the reader and to the data carrier is adjustable in strength. In case excessive power is transmitted to the data carrier, the excessive power will be lost, and in case insufficient power is transmitted to the data carrier, the data carrier will not be able to carry out its intended functions. Efficient operation of the monitoring device therefore requires a power control loop, where the data carrier sends information to the reader, enabling the reader to control the strength of the power transmission such that both excessive and insufficient power transmissions are avoided. It is also required that the strength of the power transmissions can be varied in a power efficient manner.
U.S. Pat. No. 6,073,050 discloses an efficient RF telemetry transmitter system including a first stage and a second stage. The transmitter system sends power and data to an implant device using pulse-width modulation of a high fixed frequency clock signal, e.g. a 49 MHz clock signal, within the first stage in order to provide efficient generation of an RF output signal in the second stage. Pulse-width modulation of the fixed frequency clock signal is used in order to optimally set the drive level of the output signal of the first stage. ON/OFF keying, or another modulation scheme, further modulates the clock signal with data in the first stage. The second stage includes a Class-E amplifier circuit.
It is a feature of the present invention to provide a monitoring device with improved means for inductive power transmission, hereby providing a monitoring device with reduced power consumption.
It is still another feature of the present invention to provide an improved method for controlling the strength of the inductive power transmission.